Printed circuit boards are today manufactured by a process which is almost completely automated. The circuits for a particular printed circuit board are generated using a computer aided design (CAD) machine which will not only generate a schematic of the printed circuit board but also will provide the board layout for all of the devices thereon. The printed circuit board layout information is provided to a device such as a laser plotter which exposes the artwork needed to fabricate the printed circuit board. The artwork comprises a series of transparent and opaque areas as features corresponding to the PCB devices.
However, defects may be present in the printed circuit board or the PCB artwork which would render the printed circuit board useless. It is therefore desirable to have a reference against which the artwork or the printed circuit board can be compared. These defects can have a variety of causes, including shrinkage in the artwork or failures in the fabrication process. Known systems for defect detection in printed circuit boards have often simply compared a given printed circuit board against a reference, defect free printed circuit board (i.e. golden board) to detect errors created during the fabrication process. However, this simple comparision will yield a large number of exceedingly minor defects which do not render the board unacceptable. Moreover, global changes to the features of the board caused by shrinkage of the artwork would result in all of the features being detected as defects in a system based upon a simple comparison to a golden board.
Some known optical PCB inspection systems are configured to examine inspection marks which are placed on the artwork away from the features. The dimensions of the registration marks on the printed circuit board are compared against a reference to determine the extent of shrinkage. Should the shrinkage exceed a certain magnitude, the board is considered to be defective.
The information corresponding to the features for the printed circuit board is digitized and stored on a magnetic medium, such as a magnetic tape. However, the sheer size of a resulting raster (X,Y) data file or data base requires that the data be compressed to reduce the storage volume and increase the speed of processing. A variety of compressed data base formats are found in the art, including run length encoded (RLE) format and scan line update (SLU), a format used by such companies as the American Telephone and Telegraph Company (AT&T).
In order to create an image of the PCB features, the CAD data must be translated into raster format and provided to a laser direct imager (LDI) such as the LDI 9720 or 9725 marketed by The Gerber Scientific Instrument Company, the assignee of the present invention. The comparison of devices on the printed circuit board and features on the reference image is performed by a device such as the model 1850 defect detection system also marketed by The Gerber Scientific Instrument Company.
A technique for generating a three state transitional data base (TDB) is disclosed and claimed in the commonly owned and co-pending U.S. patent application Ser. No. 508,093 entitled "A Method and Apparatus For Providing A Three State Data Base for Use With Automatic Optical Inspection Systems", incorporated herein by reference. That technique is characterized by an algorithm which creates from a two state data base a three state (black, white, gray) TDB with each feature having a single, uniform tolerance.
It would be advantageous to have a method and apparatus for creating a data base from the original CAD data for use in defect detection of printed circuit boards which would provide the capability of different tolerances for different individual printed circuit board features. The present invention is directed towards such a method and apparatus.